In a four stroke cycle engine having a fuel injector in an intake port, fuel is injected for example in synchronism with the engine rotation.
This may be done for example by injecting fuel into the intake port in the exhaust stroke of a given cylinder, the injected fuel then being aspirated into the cylinder together with air in the next intake stroke.
The amount of injected fuel is controlled so that the air-fuel ratio of the air-fuel mixture entering the cylinder is a target air-fuel ratio. To perform this control, the engine air intake amount is measured by an air flow meter installed in an engine intake passage, and the fuel injection amount is determined according to the intake air amount. To maintain suitable engine combustion conditions, increase engine output and improve the exhaust composition, the air-fuel ratio of the air-fuel mixture which is burnt in the cylinder must be precisely controlled so that it coincides with the target air-fuel ratio.
The fuel amount is determined based on the latest intake air amount, and the intake air amount is determined at least a short time in advance of the fuel injection timing. Therefore, the measured intake air amount is not strictly identical to the intake air amount which is actually aspirated into the cylinder together with injected fuel.
In particular, under transient running conditions such as during engine acceleration or deceleration, the engine rotation speed varies and consequently, the intake air amount also largely varies. During acceleration, for example, the intake air amount aspirated into the cylinder together with injected fuel in the intake stroke is larger than the intake air amount measured prior to fuel injection, and the resulting air-fuel ratio is lean. If the injection amount is not increased in such a case, the engine combustion conditions will depart from the ideal, and the expected engine output will not be obtained.
Tokko Hei 7-6422 published by the Japanese Patent Office in 1995 concerning air-fuel ratio correction under transient conditions, predicts a change value of the intake air amount occurring from fuel injection to when the intake valve closes, and corrects the fuel injection amount based on this value.
The fuel injection period is generally determined as follows. First, an injection end timing is determined so that all of the injected fuel is aspirated into the cylinder in the next intake stroke and an injection start timing is determined so that injection of a predetermined fuel amount is completed at this timing. The injection end timing is set to, for example, 20 degrees prior to exhaust top dead center.
The fuel injection amount is directly proportional to the length of the fuel injection period, and the fuel injection period becomes long when a large fuel injection amount is required such as when the engine is at low temperature. As a result, the fuel injection start timing is earlier, and the interval from the injection start timing to when the intake air valve closes increases. During this interval, injection of the determined injection amount has already started, so even if the intake air amount increases during this interval, the amount of injected fuel on this occasion cannot be adjusted to cope with this increase. This is because the injection end timing is determined as described hereabove, and the injection period cannot be modified once injection has started.
This situation is identical to prior art devices wherein the change of intake air amount is predicted to increase the fuel injection amount, e.g., when variation of the intake air amount is different from the predicted amount, correction of the fuel injection amount cannot be made until the next fuel injection.
Hence even if the air amount and fuel amount actually aspirated into the cylinder do not correspond, some time is required until a correction can be made, and during this time the air-fuel ratio is lean. This causes a decline in engine acceleration performance.